Hot water boilers for production of domestic hot water or hot water for a central heating system, conventionally comprise a segmental, cast-iron heat exchanger which is typically of a somewhat rectangular configuration and which has a combustion zone below it. Fuel, such as gas or oil, and air are fed to a burner in the combustion zone and there burnt so that the hot combustion gases rise generally upwardly through the heat exchanger, to be discharged, typically, at the top or at the back of the latter. In such a heat exchanger, the water passages may follow any desired or convenient path, usually partly horizontal and partly vertical.
Boilers of this conventional kind have been made in large quantities, down to very small sizes. In particular, small boilers for mounting on a wall are used in small houses and flats quite extensively, to provide the modest amount of hot water needed for supplying the radiators in two or three rooms and for supply to the hot water taps of the dwelling. Whilst such boilers are found to perform, in general, quite satisfactory, their manufacturing cost is higher per kilowatt of power output than that of larger units having a higher power output. This is because many factors in the manufacturing cost, as is well known, either do not vary with the power output of the unit being manufactured, or else are not proportional to its power output.
Boilers of conventional construction, having generally-rectangular heat exchangers of cast-iron, are in use having power outputs of the order of 0.42 megajoule/hr (40,000 btu per hour), but it is not economic to manufacture such boilers for outputs very much smaller than this.
However, provision of a boiler having a substantially greater heating capacity than the system calls for is generally wasteful, particularly in that such a boiler will tend to use more fuel that it need do. At the same time, there are many situations where boilers having a heating or power output capacity of about, or substantially smaller than, 0.3 megajoule/hr could be used with advantage. Examples of such situations include one-room or two-room flats, very small terrace or semi-detached houses. This is particularly so since recent escalations in building costs have driven developers of property to build smaller and smaller dwellings, in which space for any kind of appliance is scarcer than in the past, but where nevertheless there is a requirement for central heating as well as hot water for other purposes.
Reduced heating capacity is not, however, confined to small buildings, but is becoming possible more and more in buildings of all sizes, old and new, as the application of modern thermal insulation techniques reduces very substantially the heat losses and therefore the heating requirement.
Another consideration is that of size. A central heating system having a small heat input requirement will in general be installed in a place where space is limited, such as a small flat or apartment. Although the recent advent of wall mounted boilers has to a certain extent alleviated the problem of finding space for the boiler, in that it no longer needs to be placed on the floor, wall space may also be difficult to find in a small room. This is particularly so since any boiler requires a certain amount of free space around it to allow proper circulation of air for safety reasons. Again, since the boiler requires to be installed where a proper flue can be fitted, this in practice usually means that the choice of wall is limited to an outside wall where a suitable flue, either of the balanced type or otherwise, can be arranged.
It is thus desirable--and particularly for boilers of the smaller power output ranges--that the boiler, including its casing and accessories such as control unit, pump (if any), and pipe connections, shall be as small as possible, so that the likelihood is increased that it can be found a suitable position in a small room where a flue can be conveniently placed and where cold water, and gas or oil, can easily be brought to the boiler.
Proposals for "compact" boilers of comparatively small power output have been made in the past. In one such boiler, the shell-type heat exchanger is of fabricated construction and cylindrical in shape. A gas burner is mounted coaxially in the heat exchanger shell, and combustion air is supplied to the burner by a fan. Longitudinally extending pipes, arranged on a common pitch circle, are arranged through the heat exchanger shell to serve as water passages, and the hot combustion gases flow outwardly and freely from the burner towards the circumference of the shell, there to be collected and directed to the flue. Arranged between each water pipe and the next within the shell, there is a matrix of balls, so that the hot gases, to reach the outer circumference of the shell, have to pass through these matrices. Heat exchange from the hot gases to the water in the pipes thus takes place mainly through the balls and thence to the pipe walls. This ball matrix type of boiler has many promising features, but it has not yet been found possible to develop it to a stage at which it can be competitive with a conventional boiler.